all other methods use rdlock

Submitted by:	github::onovy

In the 304 case we were already skipping it for non-zero C-L.
For 204 responses this was introduced as a side effect of 271e1c52.

Fixes:	#1823

for 5 seconds if there is a waiting list.

This allows the waiting list to drain (returning 503) quickly,
rather than one by one trying to pester the backend and only
giving up after the connect_timeout.

Related to: #1823

new improved probability of getting the VSL out.

That obscured problems in cleanup

The script lacked a logexpect -wait at the end telling it to actually
wait for the required log content.

Added a new rule to match first on an actual retry backend transaction
and match the header set/unset/set tests on that vxid.

Check if system has closefrom(2) and use that where available

represents the current limit.

Also don't call it every iteration of the loop.

Avoids memcpy/free if the buffer was dynamic.

Unbreaks distcheck.